An electric linear motion actuator driven by an electric motor includes a motion converter mechanism for converting the rotary motion of the rotor shaft of the electric motor to a linear motion of an axially movable supported driven member.
Known such motion converter mechanisms include a ball-screw mechanism and a ball-ramp mechanism. While these known motion converter mechanisms can increase power to some extent, they cannot increase power to a level necessary in an electric disk brake system.
Thus, if such a conventional motion converter mechanism is used in an electric linear motion actuator, it is necessary to additionally provide the actuator with a speed reduction mechanism such as a planetary gear mechanism to increase driving power. The addition of such a speed reduction mechanism leads to increased complexity and size of the electric linear motion actuator.
In order to avoid this problem, the applicant of this invention proposed, in JP Patent Publication 2010-65777A and JP Patent Publication 2010-90959A, electric linear motion actuators which are suitable for use in electric disk brake systems because they can increase power to a considerable degree without the need for a speed reduction mechanism, and are small in linear motion stroke.
Either of the electric linear motion actuators disclosed in JP Patent Publication 2010-65777A and JP Patent Publication 2010-90959A includes a rotary shaft rotated by an electric motor, an axially movable outer ring member, and planetary rollers mounted between the rotary shaft and the outer ring member. When the rotary shaft is rotated, the planetary rollers are rotated about their respective own axes while revolving around the rotary shaft, due to frictional contact with the rotary shaft. This causes the outer ring member to move linearly in the axial direction because a helical rib formed on the radially inner surface of the outer ring member is engaged in helical grooves or circumferential grooves formed in the radially outer surfaces of the planetary rollers.
Either of the electric linear motion actuators disclosed in JP Patent Publication 2010-65777A and JP Patent Publication 2010-90959A further includes thrust bearings mounted between the respective planetary rollers and an inner disk of a carrier rotatably supporting the planetary rollers. The thrust bearings allow smooth rotation of the planetary rollers when axial loads are applied to the planetary rollers from the outer ring member.
In either of the electric linear motion actuators disclosed in JP Patent Publication 2010-65777A and JP Patent Publication 2010-90959A, axial loads from the outer ring member are applied to each planetary roller at its portion where the helical groove or circumferential grooves are in engagement with the helical rib of the outer ring member. Thus such axial loads are unevenly applied to the planetary rollers and thus such uneven loads are applied to the respective thrust bearings too.
As a result, a large axial load is applied to a rolling element of each thrust bearing that is axially aligned with the portion of the corresponding planetary roller where axial loads are applied, and a smaller axial load is applied to a rolling element that is circumferentially farther apart from the first-mentioned rolling element. This results in uneven surface pressure distribution of each thrust bearing in the circumferential direction, increasing the possibility of uneven wear of the rolling elements and the bearing races of the thrust bearings.